This is the oldest section of the Białowieża National Park and covers an area of around 47.5 sq km, bordered to the north and west by the marshy Hwożna and Narewka Rivers, and to the east by the Belovezhskaya Pushcha National Park in Belarus.
This part of the park can only be entered in the company of an official guide.
The terrain is mostly flat, swampy in parts, and covered with mixed forest of oak, hornbeam, spruce and pine. Ancient trees reach spectacular sizes uncommon elsewhere, with spruce 50m high and oak trunks 2m in diameter; some of the oak trees are more than 500 years old. The forest is home to a variety of large mammals, including elk, roe deer, wild boar, lynx, wolves, beavers and the uncontested king of the puszcza, the bison. There are about 120 species of birds, including owls, cranes, storks, hazelhens and nine species of woodpecker.
The reserve gets pretty swampy in spring (March to April) and may be closed at times to visitors. Mosquitoes can be a problem throughout the summer, so be sure to cover your arms and legs and bring along some mosquito repellent.